According to Kelvin Inman in Live Together, Die Alone, his former partner Radzinski "made some edits" to the film.
Since he states this in direct response to Desmond's question "Why are there missing parts?", it is very probable that by edits Inman meant omissions.

It is very likely that the piece of film that was found in The Arrow by Mr. Eko was removed from the original by Radzinski, though how that piece of film found its way to The Arrow remains unclear.

It is possible that Radzinski traveled to The Arrow when he explored the Island (which he must have done, since he was the one who began the Blast Door Map of the Island)